A comparison of methods for measuring the colonisation of a root system by fluorescent Pseudomonads

Methods are described for measuring the colonisation of a radish (Raphanus sativus) root system by seedlings with rifampicin resistant fluorescent pseudomonads by dilution plating, and which would take account of differences in root morphology. Differences in the levels of pseudomonad colonisation was highly dependent on the units in which surface areas was expressed. Population levels are expressed using estimates of surface area based on root length, tap-root length and root weight. The best estimate of surface area was root length, but the most practical method was surface area calculated as a function of dry weight. This method could differentiate differences in the levels of root colonisation independent of differences in root morphology and was efficient enough to allow the routine processing of a large number of replicate root samples.